1. Field of the Invention
The present invention relates to an arrangement for shadow-casting lithography by means of electrically charged particles for the purpose of imaging structures of a mask on a substrate disposed immediately to the rear thereof, comprising a substantially point-shaped particle source and an extraction system, which produces a divergent particle beam issuing from a substantially point-shaped virtual source and comprising a means for focusing the divergent particle beam to form an at least almost parallel particle beam for the purpose of illuminating the mask and the substrate.
2. Description of the Prior Art
A lithography system of this type is disclosed for example in U.S. Pat. No. 4,757,208, which describes a system for ion lithography by means of shadow-casting projection, conventionally referred to as MIBL (--Masked Ion Broad Beam Lithography/Masked ion Beam Lithography). In this system a hydrogen or helium ion source is provided with an extraction system which comprises a magnetic sector field for the purpose of mass filtering the particles which are issuing from the source. Located in the beam direction at the rear of the sector field is an electrostatic collector lens which is constructed from two coaxial tubes, which lens collects the particle beam in a so-called crossover to form an image point of the source. This crossover lies in the object-side focal plane of a second collector lens which is likewise constructed from two coaxial tubes and which forms the above mentioned means for focusing the divergent particle beam to form a parallel beam. This parallel beam irradiates a lithography mask which is disposed directly above a substrate so that it is possible to image the structure of the mask directly on this substrate.
A further system of this type is described in the periodical Optik Optics! Vol. 51, Book 5, in an article entitled "Lithium-ion-beam exposure of PMMA layers without proximity-effect" by R. Speidel and U. Behringer in 1979. In the case of this known system instead of lenses comprising two coaxial tubes, two individual lenses are provided which are formed in each case from three coaxial ring electrodes. The individual lens disposed on the mask or wafer-side forms the means for focusing the divergent particle beam to form a parallel beam.
The solution achievable by means of such a known system is dependent upon several factors, namely upon the energy unsharpness of the particles issuing from the source and upon the virtual source size, i.e. upon the narrowest diameter of the region from which the particles from the source visibly originate. Furthermore, the quality of the imaging is limited by the imaging error of the collector lenses, in particular of the second lens used to produce the parallel beam. If the particle beam is not exactly parallel, or rather is slightly convergent or divergent, in addition the inaccuracy of the distance between the mask and the substrate appears as an imaging error. Further contributory factors are also the deviations from flatness of the substrate or the mask.
The Austrian application A 259/95 of the applicant dated 13.2.1995 describes a special lens assembly for the purpose of producing an almost parallel beam which is characterised by particularly few imaging errors. In the case of this arrangement a plurality of coaxially disposed ring electrodes are provided in sequence and their potentials are such that a constant accelerating field is formed at least in sections within the beam cross-section in the beam direction. If a divergent beam of charged particles is now directed into this accelerating field, then the individual particle paths are deflected, by virtue of the accelerating effect of this field, onto a parabolic path, wherein after travelling over a certain distance an almost parallel beam is produced, which can be used to image structures of a mask onto a substrate.